Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species

Abstract The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1‐dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch‐Buttenberg‐Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4‐tetramorpholino‐1‐buten‐3‐yne (4), which was used for the synthesis of four‐membered cyclic bent allene (CBA) transition‐metal complexes of the type [(CBA)MLn] (5‐7; MLn=AuCl, RhCl(COD), RhCl(CO)2; CBA=1,3,4,4‐tetramorpholino‐1,2‐cyclobutadiene; COD=1,5‐cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4‐tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4‐tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C4N4 core as shown by X‐ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.


Introduction
1,2-Diaminoalkynes (diaminoacetylenes, DAAs) represent an electron-rich and highly reactive class of alkynes which, in contrast to tamed derivatives, such as 1-amino-and 1amidoalkynes, [1] has received comparatively little attention to date. [2] This disregard is quite surprising, since Viehe and Reinstein reported on the synthesis of the first representative, bis(diethylamino)acetylene, as early as 1964; [3] however, it can be ascribed to the rather laborious nature of the previous synthetic protocols, which generally involved the generation of halogen-substituted alkynes and their treatment with several equivalents of alkalimetal amides. [4] Better access to a series of DAAs of type I (Figure 1) was granted by the protocol introduced by our group in 2010, [5] which involved the bromination of 1,1-ethenediamines, followed by the treatment of the resulting 2,2-dibromo-1,1-ethenediamines with nBuLi to form DAAs, including Ia-Ic, through a Fritsch-Buttenberg-Wiechell rearrangement. [6] The coordination chemistry of these alkynes, [2] predominantly dipiperidinoacetylene (Ia), towards transition metals [7] and main group elements [8] has been studied, and most of their reactivity can be attributed to their latent diamino-dicarbene nature. [9] Different reactivity was uncovered by the group of Holger Braunschweig, [10] who reported the cyclization of Ia upon addition of Me 2 SnCl 2 , and similar 1,2,3,4-tetraamino-1,3-cyclobutadiene adducts II were isolated from the reactions of Ia with SnCl 2 or GeCl 2 · dioxane, respectively. [11] In this context, we also reported that Ia dimerizes under thermal conditions to the corresponding enyne, which afforded the isomeric SnCl 2 -and GeCl 2 -stabilized 1,3,4,4-tetraamino-1,2-cyclobutadiene species III under analogous reaction conditions. Related gold(I), rhodium(I), and tungsten(0) complexes of type III were also prepared (Figure 1), [11] and these species represent members of the class of so-called cyclic bent allenes (CBA), [12] which have received considerable attention from a theoretical point of view and as strong donor ligands in transition-metal chemistry and homogeneous catalysis. [13] Four-membered CBAs, however, are exceedingly rare, [14] and apart from compounds III, the only other related system had previously been generated by Guy Betrand and coworkers from a 1,3-diperidinocyclobutenylium salt by deprotonation. [15] In the preparation of the tin compound III (ML n = SnCl 2 ), we also observed that Ia can undergo a redox reaction with excess tin(II) chloride, and the tetrapiperidinocyclobutadiene dication with two trichlorostannate(I) counterions (compound IV, Figure 1) was isolated as a by-product and characterized crystallographically. [11] To the best of our knowledge, IV represents the only structurally authenticated salt containing a cyclobutadiene (CBD) dication, although related tetraamino derivatives such as V and VI had previously been reported by Siegfried Hünig [16] and Heinz Günther Viehe. [17] In the more than 120-year history of the search for stable carbocations, [18] CBD dications have received surprisingly little experimental attention, with the exception of the Nobel laureate George A. Olah, whose group described the generation and NMR spectroscopic characterization of the dicationic species C 4 Me 4 2 + , C 4 Ph 4 2 + , cis-C 4 Ph 2 H 2 2 + , and cis-C 4 Ph 2 F 2 2 + in superacidic media in the temperature range between À 78 and À 60°C. [19] Although classical Hückel rule considerations suggest square-planar geometries for these species and thus D 4h symmetry in case of C 4 H 4 2 + , [20] theoretical calculations predict folded D 2d geometries for the dications C 4 R 4 2 + (R = H, Me), which has been ascribed to the relief of destabilizing in-plane 1,3-interactions and subsequent stabilization of the puckered form by orbital mixing and orbital reorientation. [21][22][23][24][25] In case of C 4 Ph 4 2 + , C 4 F 4 2 + and C 4 (CN) 4 2 + , however, ring planar minimum geometries (dihedral angle of 0°) were theoretically predicted due to possible delocalisation of the positive charge on the substituents. [26] Accordingly, planar (C 4h ) geometries were recently confirmed for the dications in the salts [C 4 (OH) 4 ][MF 6 ] 2 · 2HF (M = As, Sb) by single-crystal X-ray structure analyses, [27] which is in line with earlier predictions for diprotonated squaric acid C 4 (OH) 4 2 + . [28] A planar geometry was also predicted for the tetraaminocyclobutadiene dication, C 4 (NH 2 ) 4 2 + , based on semi-empirical quantum chemical calculations, [29] in contrast to the experimentally observed slightly puckered structure established for the tetrapiperidino derivative IV. [11] With this contribution, we would like to introduce dimorpholinoacetylene (3, Figure 1) as a new member of the still small DAA family by reporting on its synthesis and characterization, including an X-ray crystal structure analysis. Furthermore, the reactivity of 3 has been investigated, with its dimerization to the corresponding 1,1,2,4-tetramorpholino-1buten-3-yne (4) allowing access to 1,2-cyclobutadiene metal complexes, while its protonation and subsequent oxidation with bromine affords the tribromide salt [C 4 {N(CH 2 CH 2 ) 2 O} 4 ][Br 3 ] 2 (9) with a tetramorpholinocyclobutadiene dication, that has a planar C 4 ring in the solid state.

Synthesis and characterization of dimorpholinoacetylene and its derivatives
Reaction of N,N-dimethylacetamide dimethyl acetal with an excess of morpholine at elevated temperature (110-150°C) afforded 1,1-dimorpholinoethene (1) in 80 % yield as an orange solid in analogy to previous protocols (Scheme 1). [30] The 1 H NMR spectrum (in CDCl 3 ) exhibits the expected two triplets ( 3 J H-H = 4.6 Hz) at 2.86 and 3.68 ppm for the NCH 2 and OCH 2 hydrogen atoms together with a singlet at 3.30 ppm for the olefinic CH 2 group. In the 13 C{ 1 H} NMR spectrum, the signals at 70.4 (C = CH 2 ) and 162.3 ppm (C = CH 2 ) are assigned to the olefinic carbon atoms, indicating the strong polarization of the carbon-carbon double bond. The subsequent bromination of 1 in dichloromethane (DCM) in the presence of triethylamine proceeded cleanly and afforded the dibromoketene aminal 2 as a light-yellow solid in 82 % after evaporation and extraction with toluene. The 1 H NMR spectrum (in C 6 D 6 ) exhibits the two expected triplets ( 1 J H-H = 4.6 Hz) for the remaining CH 2 groups at Scheme 1. Synthesis and dimerization of dimorpholinoacetylene (3).

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2.71 and 3.31 ppm, while the olefinic carbon atoms give rise to 13 C NMR signals at 55.2 (CBr 2 ) and 153.6 ppm (CN 2 ). It should be noted that 2 should not be stored in solution for prolonged time, since decomposition and formation of the hydrobromide 2 · HBr (S1) was repeatedly observed (see the Supporting Information for an X-ray crystal structure of S1 · CHCl 3 ).
To trigger the Fritsch-Buttenberg-Wiechell rearrangement and formation of DAA 3, the dibromide 2 was initially treated with n-butyllithium (nBuLi) in toluene solution, however, 3 was isolated in only small yield up to 20 %. The observation of significant amounts of a poorly soluble precipitate raised the suspicion that a lithium coordination polymer might have formed through the presence of the morpholine oxygen atom. Therefore, two equivalents of 1,4-dioxane were added to the reaction mixture, and indeed, 3 could be isolated in satisfactory yield (60 %) as a colorless solid after recrystallization from THF/ n-hexane. The 1 H NMR spectrum of 3 shows the two expected multiplets for the morpholine units, while the 13 C NMR signals of the acetylenic carbon atoms are found at 73.9 ppm, in good agreement with the chemical shifts reported for Ia (74.8 ppm), Ib (75.0 ppm), and Ic (77.5 ppm). [5] The molecular structure of 3 could be determined by singlecrystal X-ray diffraction analysis ( Figure 2). Overall, the structural parameters are similar to those established for DAA Ib, [5] with the molecule displaying a linear N1À C1À C2À N2 axis with N1À C1À C2 and N2À C2À C1 angles of 178.3(1)°and 177.2(1)°, respectively. The carbon-carbon triple bond lengths of 1.199(1) and 1.206(2) Å in the ynediamines 3 and Ib are almost identical, and a similar range has also recently been reported for a series of ynediamides. [31] Noteworthy, the two morpholino units are highly twisted and adopt nearly perpendicular orientations to each other with a dihedral angle of 82.6(2)°between the C1À N1À O1 and C2À N2À O2 planes. This conformation is consistently found in ynediamines [5] and ynediamides [31] and was also calculated to be more favorable for the parent DAA H 2 NC�CNH 2 . [32] The nitrogen atoms in 3 display distinctly trigonal-pyramidal environments with angle sums of 339.5°(N1) and 338.1°(N2), which excludes significant π interaction with the CÀ C triple bond.
We recently discovered that dipiperidinoacetylene (Ia) dimerizes when heated in pure form at elevated temperature to form the corresponding 1,1,2,4-tetrapiperidino-1-buten-3-yne in almost quantitative yield. [11] Based on DFT calculations, we were able to propose a plausible uncatalyzed mechanism for the dimerization of Ia that involves a 1,4-dicarbene intermediate and a subsequent 1,3-migration of one piperidino group. [11] Since dimorpholinoacetylene (3) is a solid, its dimerization was accomplished by heating a concentrated toluene solution at 110°C for 24 h, resulting in a complete conversion into 1,1,2,4tetramorpholino-1-buten-3-yne (4), which was isolated as a beige solid. Its 1 H NMR spectrum shows eight multiplets in the range 2.5-3.8 ppm for the NCH 2 and OCH 2 hydrogen atoms of the four inequivalent morpholine rings, while the quaternary carbon atoms of the C=CÀ C�C chain give rise to four 13 C NMR signals at 155.4, 101.4, 58.5, and 101.1 ppm, which is in excellent agreement with the values reported for the corresponding piperidino derivative (157.8, 102.6, 59.9 and 101.8 ppm). [11] Crystals suitable for X-ray diffraction analysis were obtained from a saturated n-hexane solution, and the resulting molecular structure is shown in Figure 3. The asymmetric unit contains two independent molecules, which mainly differ in the orientation of the morpholino substituent in the 4position, which bears the enyne substituent in the axial position only in molecule 1 (at N4 in Figure 3). The carbon-carbon bond lengths within the C=CÀ C�C chain are 1.375(3)/1.362(4) Å (C1À C2), 1.421(4)/1.423(4) Å (C2À C3), and 1.210(4)/1.213(4) Å (C3À C4), which again agrees well with the values reported for the piperidino congener. [11] Likewise, both independent molecules are markedly twisted as for instance indicated by N2À C1À C2À N3 torsion angles of 25.7(4)°(molecule 1) and 24.8(4)°(molecule 2).
The ability of enyne 4 to form cyclic bent allene (CBA) complexes in the presence of transition metals was investigated by treatment with (THT)AuCl (THT = tetrahydrothiophene) in THF solution. The initial suspension became clear, followed by precipitation of the gold(I) complex 5, which was isolated as a colorless solid in 82 % yield by filtration (Scheme 2). The formation of a cyclic CBA ligand was confirmed by 13 C NMR spectroscopy with three signals for the ring carbon atoms at  178.1 (C2/C4), 127.3 (C1), and 95.6 ppm (C3). It is noteworthy that the signal for the metal-bound carbon atom C1 is found at significantly higher field compared to conventional NHC-gold complexes such as [(IMes)AuCl] (173.4 ppm, IMes = 1,3-bis(2,4,6trimethylphenyl)imidazolin-2-ylidene) and [(SIMes)AuCl] (195.0 ppm, SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene). [33] X-ray diffraction analysis provided the molecular structure of 5 ( Figure 4). Since the molecule resides on a crystallographic C 2 axis passing through the atoms ClÀ AuÀ C1À C3, the gold coordination sphere is perfectly linear (C1À AuÀ Cl = 180°), and the four-membered ring as well as the atoms Au, Cl, N1 and N1' are coplanar to within 0.005 Å. At 1.985(2) Å, the AuÀ C1 bond length is very similar to those established for NHC gold(I) complexes, compare 1.998(5) Å in [IMes)AuCl] and 1.983(4) Å in [(SIMes)AuCl]. [33] Noteworthy, the possibilty to introduce more than one CBA ligand was revealed by isolation of single crystals of the corresponding homoleptic gold(I) complex [(CBA) 2 Au]Cl (S2) on one occasion from dichloromethane solution (see the Supporting Information for the crystal structure of S2 · 1.5CH 2 Cl 2 ). The formation of this byproduct could also account for the slightly reduced yields observed upon metal complexation.
In a similar fashion, the rhodium(I) CBA complex 6 was isolated as a yellow solid in 56 % yield from the reaction of enyne 4 with half an equivalent of [Rh(COD)(μ-Cl)] 2 in THF solution. Bubbling CO through a THF solution of 6 afforded the cis-dicarbonyl complex 7 as a dark yellow solid in 62 % yield (Scheme 2). The carbon atoms of the four-membered CBA ligand in 6/7 gave rise to 13 C NMR signals at 179.1/180.2 (C2/ C4), 148.6/135.8 (C1), and 95.7/94.6 ppm (C3), with the metal bound carbon atoms (C2) producing dubletts with 1 J C-Rh coupling constants of 41/32 Hz (6/7). These values are in good agreement with those reported for similar bis(piperidinyl) complexes. [11,15] Likewise, the CO stretching frequencies determined for 7 of 1984 and 2064 cm À 1 (ũ av = 2024 cm À 1 ) confirm the strong donor ability of the CBA ligand. The solid-state structures of 6 and 7 could be established by X-ray diffraction analysis, and pertinent structural parameters are assembled in Table 1. The molecular structure of 6 is shown in Figure 5, while that of 7 is presented in the Supporting Information ( Figure S5). In both structures, the rhodium atoms display the expected square-planar coordination spheres, with the four-membered CBA ligands adopting perpendicular orientations with dihedral angles of 83.9(1)°(6) and 85.2(2)°(7) between the C1À C2À C3À C4 and C1À RhÀ Cl planes. The RhÀ C1 distances are 2.012(2) Å (6) and 2.053(4) Å (7) and fall within the range established for the corresponding bis(piperidinyl) systems. [11,15] In a similar fashion as described by Viehe and coworkers for the preparation of tetrakis(dimethylamino)cyclobutenediylium dibromide (VI, Figure 1) in two steps from bis(dimethylamino)acetylene, [17] the reaction of dimorpholinoacetylene (3) with triethylammonium bromide in dichloromethane afforded the cyclobutenylium bromide 8 as a beige solid in 68 % yield after evaporation and washing with ethyl acetate (Scheme 3). In the 1 H NMR spectrum, a singlet at 6.04 ppm can be assigned to the hydrogen atom at the ring carbon atom C1, which is at lower field compared to 5.10 ppm reported for the related   Table 1. [a] E = centroid C25-C26 (6), C22�O (7).

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Research Article doi.org/10.1002/chem.202202737 tetrakis(dimethylamino)cyclobutenylium bromide. [17] While no 13 C NMR data were reported for the latter compound, 8 gives rise to 13 C NMR signals at 168.8 (C2/C4), 117.7 (C3), and 67.4 ppm (C1, this signal was identified in the 1 H-13 C-HSQC 2D NMR by a correlation peak to the signal at 6.04 ppm). These values are in good agreement with those of the structurally related Sn and Ge complexes II, [10,11] indicating cyclization and formation of a four-membered ring upon protonation of DAA 3. X-ray diffraction analysis provided additional evidence for the formation of a cyclobutenylium ion, and the molecular structure of 8 reveals a nearly planar C 4 ring that is only slightly puckered with a dihedral angle of 6.1(2)°between the C2À C3À C4 and C2À C1À C4 planes ( Figure 6, top). Planarization of the nitrogen atoms N2 and N4 with angle sums of 356.7°(N2) and 359.1°( N4) and short N2À C2 and N4À C4 bond lengths of 1.318(2) and 1.303(2) Å indicate π-delocalization of the positive charge over the N2À C2À C3À C4À N4 cyanine-type moiety, whereas the morpholino rings at C1 and C3 adopt perpendicular orientations with significantly smaller angles sums of 339.4°(N1) and 343.7°(N3). The bromide counterion is well separated from the cyclobutenylium cation and lies above the C 4 plane with 3.603(2) Å from the centroid of the four-membered ring and has short contacts with the CH 2 hydrogen atoms of neighboring morpholine rings. Addition of one equivalent of bromine to a solution of 8 in dichloromethane was expected to afford the tetra(morpholino)cyclobutenediylium dication as a dibromide salt, however, mixtures of bromide and presumably polybromide salts were obtained under various reaction conditions. [34] In contrast, the reaction of 8 with an excess of bromine (3 equiv.) at 0°C led to the precipiation of the bis(tribromide) salt 9 as a yellow powder from the dichloromethane reaction solution. Careful removal of the supernatant solution and washing with dichloromethane afforded 9 in 38 % yield. This comparatively low yield can tentatively be ascribed to partial hydrolysis of 8 and/or 9 under these reaction conditions since crystals of the monocationic oxygenated side products S3 and  Table 1.  with thermal displacement parameters drawn at 50 % probability; hydrogen atoms (except for H1 in 8) and anions (Br À in 8 and 2xBr 3 À in 9) are omitted for clarity. Pertinent structural data is assembled in Table 2. S4 were isolated from the separated dichloromethane solution (see the Supporting Information for X-ray crystallographic details). 1 H and 13 C{H} NMR spectra of 9 were recorded in CD 3 CN solution, revealing the presence of four equivalent morpholino substituents as expected for a symmetric cyclobutadiene dication. Accordingly, only one lowfield 13 C NMR signal at 148.3 ppm was found, which can be assigned to the quarternary ring carbon atoms. This chemical shift agrees well with the value of 152.8 ppm reported for hexakis(dimethylamino)benzene. [35] Single crystals of 9 were obtained by layering a CH 3 CN solution on EtOAc at À 40°C, and X-ray diffraction analysis confirmed the formation of a cyclobutadiene dication with two tribromide counterions. The C 4 N 4 unit is almost perfectly planar, and the carbon atoms C1À C4 are coplanar to within 0.009 Å, with a maximum deviation of 0.044 Å of the nitrogen atoms from this plane. The atoms N1À N4 exhibit trigonal-planar environments with angle sums close to 360°and short carbonnitrogen bond lengths of 1.312(7)-1.327(7) Å, which reveals significant π-interaction; however, the morpholino units are slightly twisted by about 30°, presumably for steric reasons. The carbon-carbon bond lengths in the four-membered ring range from 1.450(8) to 1.462(6) Å, falling between the typical values of 1.54 and 1.34 Å assigned to carbon-carbon single and double bonds, respectively. Similar values are also commonly found in compounds containing the squarate dianion (C 4 O 4 2À ). [36] Finally, the tribromide counterions exhibit strictly linear geometries, i. e. Br1À Br2À Br3 = 178.29(3)°and Br4À Br5À Br6 = 179.11(3)°, with typical BrÀ Br distances of Br1À Br2 = 2.5764(9) Å, Br2À Br3 = 2.5284(9) Å, Br4À Br5 = 2.5535(9) Å, Br5À Br6 = 2.5534(9) Å. [34] In the crystal structure, the cations and anions form alternating layers with each Br 3 À ion making short contacts of approximately 3.5 Å to the C 4 ring of one dication, which is just slightly below the sum of the crystallographic van der Waals radii (3.6 Å). [37] A packing diagram is provided in the Supporting Information ( Figure S8).

Computational study of cyclobutadiene dications
According to Hückel's 4n + 2-rule, [38] cyclobutadiene dications should be aromatic systems, as they fulfil this rule with n = 0, which would result in a planar C 4 X 4 2 + unit. However, this planarity has never been demonstrated experimentally for the dicationic systems C 4 H 4 2 + and C 4 Me 4 2 + , and theoretical calculations yielded nonplanar ground-state structures with D 2d instead of D 4h symmetry. [21][22][23][24][25] The non-planarity of the system can be explained by considering the resonance structures of the parent cyclobutadiene dication. Thus, there are four structures of type IA in which the two positive charges are located at adjacent carbon atoms, rendering these structures unstable due to charge repulsion. The two resonance forms of type IB have the charges further apart but are diradicals. To stabilize the diradical, the system folds to form partial bonds between the carbon atoms in the 1,3-and 2,4-positions, respectively (Scheme 4). Note that despite the non-planarity, the two π-electrons are fully delocalized over the four carbon atoms, resulting in an aromatic system. This also explains why the planar cyclobutadiene dication (a transition state between the two non-planar structures) is less aromatic than the nonplanar structure since the destruction of the 1,3-interactions reduces the delocalization (see below).

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Research Article doi.org/10.1002/chem.202202737 significantly folded and exhibits a dihedral angle of 17.1°. The structural parameters of the C 4 (NMe 2 ) 4 2 + system are very similar to those in isomer a with twisted orientations of the amino substituents and a planar C 4 ring. In contrast, the parent amino system C 4 (NH 2 ) 4 2 + exhibits consistently shorter bonds and a perfectly planar D 4h geometry, in agreement with semi-empirical calculations. [29] For the dications C 4 Me 4 2 + and C 4 H 4 2 + , puckered structures with dihedral angles of 20.2°and 30.5°w ere obtained, in agreement with previous calculations. [26] When assessing aromaticity, one must be aware that substituents, especially π donors (and π acceptors) are not electronically benign. [39] Taking C 4 (NH 2 ) 4 2 + as an example: The system is completely planar, so there is a perfect conjugation between all p π orbitals -four on the carbon atoms and four on the nitrogen atoms. The number of π electrons is 10, delocalized over the entire π system. Looking only at the fourmembered ring gives an incomplete picture. Even using the fragment molecular orbital (FMO) approach, it is clear that π donors/acceptors change the electron density at the ring and its aromatic properties. In the case discussed here, electron donors cause the cyclo-C 4 system to contain more than 2 electrons, reducing its aromaticity by moving towards a 4πelectron (antiaromatic) system. Accordingly, two contradictory stabilizing effects must be considered for the dications C 4 (NR 2 ) 4 2 + : on the one hand, aromaticity, which is maximal when two electrons are delocalized, and on the other hand, the positive charge, which is further delocalized and stabilized when electron density is transferred from the substituents to the ring. In fact, the NBO charges indicate almost equal localization of the two positive charges on the C 4 ring and on the four amino substituents, for example, + 1.08 and + 0.92 for C 4 (NR 2 ) 4 2 + (R = H, Me; see the Supporting Information). In this regard, the two isomers a and b of the dication C 4 (Morph) 4 2 + might be regarded as a manifestation of this "dilemma" between optimizing charge stabilization and aromaticity in planar (a) and folded (b) systems, respectively.
The Nucleus Independent Chemical Shift (NICS) method has become the most widely used computational method for the identification and quantification of aromaticity. [40] There are several NICS-based methods, all are based on the idea of a ghost probe (no nuclei, no orbitals, no electrons) placed anywhere, and its absolute shielding, which is a manifestation of the induced magnetic field, is computed, usually by the Gauge-Independent Atomic Orbital (GIAO) method. [41] The most common NICS-based method is NICS(1) zz , where the probe is placed 1 Å above the centre of the molecular plane and only the out-of-plane component ZZ (assuming that the system is in the XY plane) of the shielding tensor is considered. The most refined NICS-based method, however, is based on distilling the contribution of only the π molecular orbitals to the out-of-plane part of the shielding tensor (NICS π,zz ) at 1 or 1.7 Å above the molecular plane, or as an integral of NICS values from the molecular plane to infinity (∫NICS π,zz ). [42] Table 3 summarizes the NICS(1) π,zz of the different C 4 X 4 2 + derivatives. While the parent cyclobutadiene dication is as aromatic as benzene (Entry 1), it loses a large part of its diatropicity upon planarization (see above, Entry 2). The tetramethyl-substituted system shows ca. 50 % decrease in diatropicity, suggesting a rather effective electron donation from the Me groups into the ring (Entry 3). The relatively short C ring À C Me bond length (1.452 Å), the small HÀ C Me À C ring angle of the hydrogen perpendicular to the ring (105.6°) and the longer CÀ H bond of this hydrogen atom (1.111 Å vs. 1.092 Å for the other two CÀ H bonds) suggests efficient hyperconjugation. As in the parent dication, about 50 % of the tropicity is lost on planarization (Entry 4). Since conjugation is more efficient than hyperconjugation, more electron density is transferred from the amino (NR 2 ) compared to methyl (Me) groups. This is reflected in a further reduction of the NICS(1) π,zz values in the planar C 4 (NR 2 ) 2 + (R = H, Me) systems to values that may be considered as non-aromatic (Entries 5 and 6). Indeed, the NICS-XY scan of C 4 (NH 2 ) 4 2 + (Figure 7) shows that the major diatropicity is located on the nitrogen atoms, while the ring current is minimal (and diatropic) at the four-membered ring. As mentioned above, the competing effects, charge stabilization and aromaticity, are manifested in the two isomers of C 4 (Morph) 4 2 + : a, which is almost planar and therefore non-aromatic (Entry 7) and b, which is folded and exhibits slightly higher diatropicity, namely, the C 4 ring is about 19 % aromatic with respect to the parent C 4 H 4 2 + system (Entry 8). The electron donation to the fourmembered ring and the diatropic currents at the nitrogen atoms are also experimentally evidenced by a ca. 0.9 ppm downfield shift of the NCH 2 hydrogen atoms in 9 relative to 3 (see the Experimental Section). Apparently, at least 50 % of the delocalization (aromaticity) at the 4-membered ring in all C 4 R 4 2 + Table 3. σ-only-NICS(1) π,zz of some dicationic cyclobutadiene derivatives; CMO-NICS(1) π,zz (available only for completely planar systems) are given in parentheses.

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Research Article doi.org/10.1002/chem.202202737 derivatives result from the 1,3-interactions (Scheme 4) since these are lost upon planarization.
To obtain a clean π contribution based on orbital analyses, the π system must be orthogonal to the σ system, which is possible only in planar systems. Thus, Figure 8 shows the current density of planar C 4 H 4 2 + , C 4 Me 4 2 + , and C 4 (NH 2 ) 4 2 + . Although the tropicity of planar C 4 H 4 2 + and C 4 Me 4 2 + are reduced with respect to the optimized folded structures, the trend is very clear: The parent C 4 H 4 2 + shows a diatropic circuit at the ring. The C 4 Me 4 2 + shows local diatropic circuits on the Me groups, a global diatropic current and a local current at the four-membered ring, which is significantly reduced relative to the parent system. This trend, which is also reflected in the NICS(1) π,zz values ( Table 3), continues with C 4 (NH 2 ) 4 2 + , exhibiting a global current, local currents at the NH 2 groups and only a negligible local current at the four-membered ring, in accordance with the NICS-XY scan (Figure 7).

Conclusion
With the synthesis and characterization of dimorpholinoacetylene (3) an important new derivative of this interesting class of electron-rich diamminoacetylenes (DAA) has been established, and the tendency of these species to undergo dimerization has been confirmed by isolation and structural characterization of the enyne 1,3,4,4-tetramorpholinobut-1-yn-3-ene (4). The reaction of the latter with transition metals provided access to complexes containing four-membered cyclic bent allene (CBA) ligands, which represent an interesting new class of potential ancillary ligands for applications in homogeneous catalysis. Furthermore, the reaction of DAA 1 with triethylammonium bromide followed by oxidation with bromine afforded tetra(morpholino)cyclobutenediylium bis(tribromide) (9). X-ray diffraction analysis provided the molecular stucture of a tetraaminocyclobutadiene dication, namely C 4 (Morph) 4 2 + in 9, with an almost perfectly planar C 4 ring, which agrees with the ground-state geometries predicted for this and related tetraaminocyclobutadiene dications C 4 (NR 2 ) 4 2 + (R = Me, H) by density functional theory (DFT) calculations, in contrast to the puckered geometry of the parent C 4 H 4 2 + system. Detailed studies employing advanced NICS methods were conducted to assess the issue of aromaticity for these species, which reveal that the πdonating ability of the amino substituents is responsible for the observed planarity and for the delocalization of the positive charge over the nitrogen atoms, which perturbs the aromatic ring current. It is important to realize that -counterintuitivelythe cyclobutadiene dication derivatives are more aromatic when they are puckered rather than planar. The presence of πdonating amino substituents, however, leads to planarization at the expense of aromaticity. In principle, this conflict manifests itself in the two computationally derived, almost isoenergetic isomers of C 4 (Morph) 4 2 + , of which the planar isomer a, as structurally authenticated in 9, can be described as nonaromatic, while the puckered isomer b can be considered slightly aromatic with a ring current about 20 % of that in benzene. [39] Experimental Section Material and Methods: Unless otherwise noted, all reactions have been performed in dry argon atmosphere in a Glove Box (MBraun 200B) or using a high vacuum line with common Schlenk techniques. Elevated temperatures were achieved by a silicone oil bath, whereas low temperatures were provided by a sodium chloride/ice bath (0°C). Starting materials were purchased from commercial sources (TCI, abcr, Merck, Roth, Alfa-Aesar) and if necessary purified by conventional techniques or dried over CaH 2 . Solvents were dried either over Na/benzophenone, CaH 2 (chlorinated solvents) and distilled or by a SPS (solvent purification system) with subsequent degassing ("freeze-pump" or ultrasonic bath). They were stored in dry argon atmosphere over molecular sieves (4 Å). NMR spectra were recorded on Bruker AVII300, AVIIHD300 (300 MHz) or AVII500 (500 MHz) and referenced to the residual undeuterated solvent signal ( 1 H NMR) or the solvent signal itself ( 13 C{ 1 H} NMR), respectively. Coupling constants (J) are indicated in Hertz (Hz) and splitting patterns are specified as multiplett (m), singlet (s), doublet (d) or triplet (t).